1. Field of the Invention
The present invention relates to an image forming apparatus such as an electrophotography system copying machine, a printer, and so forth.
2. Description of the Related Art
In recent years, with regard to image forming apparatuses such as electrophotography system copying machines, printers, and so forth as well, there has been demand for reduction in power consumption. In particular, of devices to be mounted on image forming apparatuses, a fixing apparatus configured to heat a recording material supporting a toner image and to fix the toner image on the recording material is one of the most power consuming devices, and accordingly, there has been great demand for power reduction demand for fixing apparatuses.
However, in the event that heat quantity lacks at the time of fixing a toner image on a recording material in a fixing apparatus, an image defect such as cold offset or the like may occur. Accordingly, heretofore, even in the event that toner weight per unit area is the maximum, fixation temperature has been uniformly set to the minimum temperature where fixing is enabled, and also, no cold offset occurs. Accordingly, excessive power has been supplied to the fixing apparatus in the event that toner weight per unit is been small, and accordingly, waste of power has occurred.
Therefore, in order to reduce waste of power such as described above, it can be conceived to adjust power to be supplied to a fixing apparatus according to weight per unit area of toner supported on a recording material. In Japanese Patent Laid-Open No. 2006-154413, there has been disclosed an image forming apparatus configured to change heater control temperature according to weight per unit area of unfixed toner. The image forming apparatus according to Japanese Patent Laid-Open No. 2006-154413 can reduce power consumption by predicting weight per unit area of unfixed toner from image data, and correcting heater control temperature.
However, with the image forming apparatus according to Japanese Patent Laid-Open No. 2006-154413, in the event that weight per unit area of toner of recording materials to be consecutively printed changes greatly from page to page, temperature at the nip portion does not follow the weight of toner, and an image defect such as cold offset or the like may occur.
For example, cold offset may occur immediately after printing a recording material of which weight per unit area of toner is small, and in the event of printing a recording material of which weight per unit area of toner is great. This cold offset occurs because temperatures of a film and a pressing roller does not increase up to temperature according to toner weight per unit area of a toner image on a recording material, and this is apparent at a front edge of a recording material. At the time of performing printing in a state in which the target temperature of the heater is low, the temperatures of the film and pressing roller are kept in a low state. Even when suddenly increasing the target temperature of the heater from a state in which the temperatures of the film and pressing roller are low, the temperatures of the film and pressing roller somewhat having heat capacity may not rise with good responsivity.